The present invention relates to a non-volatile ferroelectric capacitor-based memory circuit, and more particularly related to a non-destructive ferroelectric capacitor-based memory circuit method.
Integrated circuit memory devices containing ferroelectric memory cells therein typically contain ferroelectric capacitors to store data. As will be understood by those skilled in the art, ferroelectric capacitors typically comprise a pair of electrodes with a layer of ferroelectric material therebetween having hysteresis characteristics. The ferroelectric material can typically be polarized in a first or a second opposite polarization state and this state is nonvolatile inasmuch as the application of a zero potential bias across the pair of electrodes will not result in a change or elimination of the state. The first and second polarization states can be utilized to reflect the storage of logic 0 and logic 1 data signals, respectively.
FIG. 1 illustrates a hysteresis curve of ferroelectrical material, wherein the abscissa represents the field voltage applied to the material and the ordinate represents the polarization of the material. If a capacitor is formed using a ferroelectric material between its plates, because of the hysteresis curve, the flow of current through the capacitor will depend on the prior history of the voltages applied to the device. If a ferroelectric capacitor is in an initial state on which a zero volt charge is applied, point A or point D may indicate polarization. Assuming that point A in FIG. 1 indicates polarization, a positive voltage which is greater than the coercive voltage (referring to point B in FIG. 1) is applied across the capacitor. The capacitor will then conduct current and have a new polarization state (referring to point C in FIG. 1). When the applied voltage is removed, the ferroelectric capacitor will maintain the same polarization state as shown at point D instead of returning to the state as shown at point A. A positive voltage continuously applied across the capacitor will cause a little change in the polarization. However, enough negative voltage will cause the polarization to vary from point D to point E as indicated in FIG. 1. Once the negative voltage is removed from the capacitor, the ferroelectric capacitor will maintain the same polarization state and the curve moves to point A. Therefore, point A and point D respectively represent two different logical states when zero volt is applied across the capacitor.
Conventional ferroelectric memory circuits include a plate line, a bit line, and a number of memory cells comprising a capacitor and a transistor connected between the plate line and the bit line. A particular memory cell is accessed by driving one of the transistors with a selected word line signal, and then driving the plate line with a pulse, generally of the supply voltage magnitude. If one polarization state is stored in the capacitor, then an electrical charge of nominal magnitude is transferred from the capacitor to the bit line. On the other hand, if the ferroelectric capacitor is initially stored the other polarization state, a substantially larger electrical charge is transferred to the bit line. Sense amplifier circuits are utilized to sense the bit line voltage, and thus the amount of charge transferred thereto during the read operation, and thereby determine the polarization state initially stored in the ferroelectric capacitor. The smaller amount of charge transferred from the ferroelectric capacitor during the read operation to the bit line does not involve a change in the polarization state of the capacitor itself. Hence, the reading of the ferroelectric capacitor in this state is nondestructive. However, when the read operation of a ferroelectric capacitor is accompanied by the substantially larger transfer of electrical charge to the bit line, the ferroelectric capacitor changes state from one polarization polarity to the other.
In order to circumvent this polarization change, the conventional memory circuits normally include a restore cycle for restoring the original polarization state due to the destructive readout. Although ferroelectric memory devices are characterized as being nonvolatile, and the destructive read operations can be corrected by a restoration operation, such devices are yet susceptible to problems which cannot be corrected. For example, should the power fail or be removed from a conventional ferroelectric memory device during an ongoing read operation in which the polarization states change, the ferroelectric capacitor may be in the incorrect state when power is again applied to the memory, thereby storing corrupted data. On the other hand, in those memory read operations which result in the changing or reversal of ferroelectric capacitor polarization states, the capacitors themselves undergo a phenomenon termed xe2x80x9cfatiguexe2x80x9d, which reduces the life of the capacitor. As a result of fatigue, the reliability and life of a ferroelectric capacitor is proportional to the number of times it has been read and/or written. Therefore, the reliability and life of a ferroelectric capacitor would be increased if ferroelectric capacitors could be read without reversal of the polarization states.
From the foregoing, it can be seen that a need exists for a method and circuits adapted for reading ferroelectric capacitors such that the polarization states are not destroyed or switched to the other state. A related need exists for a ferroelectric memory cell structure which can be read and which does not require a subsequent restore operation. Another need exists for a ferroelectric memory cell which can be read and which does not substantially affect the life of the capacitor due to fatigue.
The read operation cycle of a conventional ferroelectric capacitor memory circuit often involves a destructive read because the ferroelectric capacitor changes state from one polarization state to the other. In order to maintain the original data (original polarization state), the conventional memory circuit needs a restore cycle for restoring the original data. The time required for restoring data will reduce the operation speed. Those reading operation cycles which include the destructive read and restoring cycle may also result in the ferroelectric material xe2x80x9cfatiguexe2x80x9d, which reduces the life and reliability of the ferroelectric capacitor. As a result of the xe2x80x9cfatiguexe2x80x9d, the reliability and life of a ferroelectric capacitor is proportional to the number of times it has been read and written. On the other hand, if the power fails during the restoring operation period, the stored data is destroyed.
The main purpose of the present invention is to provide a kind of ferroelectric capacitor-based memory circuit which can be read and does not require a subsequent restore operation.
The other purpose of the present invention is to provide a kind of operation method of the ferroelectric capacitor-based memory circuit memory in which the polarization states are not destroyed or switched to the other state.
The another purpose of the present invention is to provide a ferroelectric memory cell which can be read, in which the life of the capacitor is not substantially affected by fatigue.
In accordance with the foregoing purpose, the present invention is related to a ferroelectric capacitor-based memory circuit and operation method. The present invention comprises a plurality of word lines, a plurality of bit lines and a number of memory cells comprising two ferroelectric capacitors and a transistor. The two ferroelectric capacitors are coupled in series to form a common node between the two ferroelectric capacitors, with two poles each corresponding uniquely to only one respective ferroelectric capacitor. The two plate lines are coupled to the poles respectively. Because the ferroelectric capacitor will exhibit different capacitance according to the polarization, the polarization states of the two ferroelectric capacitors are set corresponding to the data to be stored. One of the plate lines is grounded while a voltage less than the coercive voltage is applied to the other plate line. Then the data stored in the memory can be read out by sensing whether the voltage at the common node is above or below the midpoint voltage.
According to the present invention, the disclosed ferroelectric capacitor-based memory circuit and operation method are adapted for reading a ferroelectric capacitor such that the polarization states are not destroyed or switched to the other state and a subsequent restore operation is not required. Hence, the life or reliability of the capacitor is not affected by to fatigue resulting from the switch of polarization states. The present invention also discloses a simple method for writing the polarization states of a memory cell.